1. Field of the Invention
The present invention relates to an ion source of the so-called Bernus type having a structure in which a filament and a reflector are provided within a plasma production vessel and a magnetic field is applied in a direction of connecting the filament and the reflector, and operation method applying the ion source, and more particularly to a device which enhances the ratio of molecular ions in an ion beam.
2. Description of the Related Art
One example of the ion source of this kind was disclosed in Japanese Patent Unexamined Publication No. Hei. 11-339674(JP-A-11-339674), for example. This will be described below with reference to FIGS. 3 and 4.
This ion source comprises a plasma production vessel 2 into which an ion source gas is introduced from a gas inlet opening 6 serving as an anode, a U-character shaped filament 8 provided through a wall face of the plasma production vessel 2 on one side of this plasma production vessel 2, and a reflector 10 (reflecting electrode) provided opposite the filament 8 on the other side of the plasma production vessel 2. Reference numerals 24 and 30 denote insulators.
On the wall face of the plasma production vessel 2, a long ion lead-out slit 4 is provided in a direction of connecting the filament 8 to the reflector 10. In a vicinity of an exit of this ion lead-out slit 4, a lead-out electrode 14 is provided to lead out an ion beam 16 from within the plasma production vessel 2 (more specifically from a plasma 12 produced therein).
Outside the plasma production vessel 2, a magnet 18 is provided to generate a magnetic field 19 in a direction of connecting the filament 8 to the reflector 10 within the plasma production vessel 2. The magnet 18 is an electromagnet, for example, but may be a permanent magnet. The magnetic field 19 may be an inverse direction to that as shown in the figure.
The orientation of the filament 8 is indicated as a matter of convenience to clarify the connection with a filament power source 20 in FIG. 3. In practice, a face containing the filament 8 bent like the U-character is arranged to be substantially parallel to the ion lead-out slit 4, as shown in FIG. 4.
The filament power source 20 for heating the filament 8 is connected to both sides of the filament 8. Between one end of the filament 8 and the plasma production vessel 2, an arc power source 22 is connected to apply an arc voltage VA between the filament 8 and the plasma production vessel 2, causing an arc discharge between them, and ionizing an ion source gas to produce a plasma 12.
The reflector 10 acts to reflect an electron emitted from the filament 8, and may be kept at a floating potential without connecting anywhere as in an illustrated example, or at a filament potential by connecting to the filament 8. If such reflector 10 is provided, an electron emitted from the filament 8, under the influence of a magnetic field 19 applied within the plasma production vessel 2 and an electric field of the arc voltage VA, is reciprocating between the filament 8 and the reflector 10, while revolving in the magnetic field 19 around an axis in the direction of the magnetic field 19. As a result, the probability of collision of the electron with a gas molecule is increased to cause the ionization efficiency of the ion source gas to be enhanced, thus resulting in the higher production efficiency of the plasma 12.
Conventionally, in order to enhance the production efficiency of the plasma 12 by increasing the life of an electron emitted from the filament 8 till collision against the wall face of the plasma production vessel 2, it is common that the magnetic flux density B of the magnetic field 19 within the plasma production vessel 2 is set up so that the Larmor radius R (see Numerical Expression 2 as will be described later) of the electron in the magnetic field 19 is smaller than the shortest distance L from the most frequent emission point 9 located almost at the tip center of the filament 8 to the wall face of the plasma production vessel 2.
As shown in FIG. 1, an ion beam 16 led out of the ion source contains a molecular ion (e.g., P2+, As2+), which is an ion like a molecule, besides a monatomic ion (e.g., P+, As+). The molecular ions include, for example, a diatomic ion composed of two atoms, and a triatomic ion composed of three atoms.
The molecular ion has the following advantages over the monoatomic ion. Namely, (1) the molecular ion has enhanced transport efficiency because of less divergence than the monoatomic ion, (2) because when the molecular ion is implanted into a target, a plurality of atoms are implanted, the implantation amount (dose amount) can be obtained almost multiple times that of the monoatomic ion in the case of a same beam current, and (3) on the contrary, in the case of a same implantation amount, the molecular ion has a less beam current, thus a smaller amount of charges incident on the target, than the monoatomic ion, whereby it is expected that there is the effect of suppressing the charge-up (charging) of the target.
From such a point of view, it is preferable that the ratio of molecular ions in an ion beam is higher. Thus, it is an object of this invention to enhance the ratio of molecular ions in an ion beam.
An ion source according to this invention is set up such that supposing that the arc voltage applied between the plasma production vessel and the filament is VA[V], the magnetic flux density of the magnetic field within the plasma production vessel is B[T], and the shortest distance from a most frequent electron emission point located almost at the tip center of the filament to the wall face of the plasma production vessel is L[m], a relation of the following expression (1) is satisfied.
L less than 3.37Bxe2x88x921(VA)xc3x9710xe2x88x926xe2x80x83xe2x80x83(1) 
An operation method of an ion source according to this invention is set up to lead out an ion beam such that supposing that the arc voltage applied between said plasma production vessel and said filament is VA[V], the magnetic flux density of the magnetic field within said plasma production vessel is B[T], and the shortest distance from a most frequent electron emission point located almost at the tip center of said filament to the wall face of the plasma production vessel is L[m], the above-described expression 1 is satisfied.
Various physical collisions, molecular dissociation, or chemical reactions of electrons, ions, atoms, and molecules occur inside a plasma produced within the plasma production vessel, constantly repeating the production and disappearance of molecular ions. To prevent the produced molecular ions from being dissociated, it is effective to decrease the probability of existence of electrons having energy over several electron volts.
The Larmor radius R of electrons emitted from the filament revolving in the magnetic field within the plasma production vessel can be represented in the following expression (2). Where B and VA are as mentioned previously, m is a mass of electron, and e is a quantum of electricity.
R=Bxe2x88x921(2mVA/e)≈3.37Bxe2x88x921(VA)xc3x9710xe2x88x926[m]xe2x80x83xe2x80x83(2) 
That is, the right side of the expression 1 represents the Larmor radius R of this electron, whereby the expression 1 can be written as L less than R. If such a condition is set up, the probability that an electron having a high energy collides against the wall face of the plasma production vessel and quenches is increased, making it possible to shorten the life (existence probability) of electrons having high energy, whereby the ratio of molecular ions in a plasma can be enhanced, as described above. As a result, the ratio of molecular ions in an ion beam can be enhanced.